1. Field of the Invention
The invention is applicable to a backlight source device of a large or ultra-large LCD monitor, which needs multiple CCFLs (Cold Cathode Fluorescent Lamps) or EEFLs connected in parallel as a light source. The required high frequency power is supplied from electronic ballast or a switching power in order to protect the CCFL tubes or EEFL tubes with a single working frequency, a stable circuit, a high emitting efficiency, a high quality, and a small distortion. The invention provides a measurement and protective circuits and precise and stable voltage and current for PWM (Pulse Width Modulation) in order to supply the power to a TFT LCD TV system.
2. Description of the Related Art
When the conventional CCFLs or EEFLs are used in a backlight source for a LCD monitor, an inverter typically lights one or two CCFLs or EEFLs. When the conventional CCFLs or EEFLs are used in a large LCD monitor or a large television LCD display, five, ten or more than ten inverters have to be utilized, and the following drawbacks appear.
1. When one of more than ten CCFL tubes or EEFL tubes is damaged and cannot be lighted, the condition cannot be effectively detected, and the quality of the LCD monitor is influenced.
2. Because there are some differences between the high working frequencies of the inverters, many interference frequencies are generated in the LCD monitor, and the cost and burden for eliminating the electromagnetic interference are thus increased.
3. Because there are some differences between the output values of the high frequency voltages of the inverters, the luminances of the CCFLs or EEFLs are not the same, and the quality of the LCD monitor is influenced.
4. The cost of multiple inverters is higher than that of single electronic ballast or a single inverter.
Thus, it is an important subject of the invention to provide a CCFL tube or EEFL tube protective circuit capable of solving the above-mentioned problems of the conventional CCFLs or EEFLs used as the backlight source of the LCD monitor. In the CCFL tube or EEFL tube protective circuit, it is possible to detect one damaged tube among several tens of CCFL tubes or EEFL tubes, and the quality of the LCD monitor can be maintained using the technology of PWM precision stable voltage or current. Hence, the drawbacks of the need of multiple inverters, of the frequency interference, of the complicated structure and of the high cost can be solved.